This invention relates generally to differential amplifiers and in particular to a differential amplifier having low impedance self biasing inputs.
Differential amplifiers are highly useful because of their ability to handle a differential mode signal in the presence of the commond mode signal without adverse effects to the differential mode signal. It is often desirable to produce low impedance and ultra low impedance (less than one ohm) input differential amplifiers for current mode applications requiring wide bandwidths with frequency response down to DC. For those amplifiers having low impedance transducer devices coupled to inputs, it is desirable to provide differential isolation for such transducers allowing operation of the devices fully floating from ground and still operating near ground potential. Current mode instrumentation amplifiers also make efficient use of low input impedance amplifiers from maxium coupling efficiency between input device and amplifier.
One example of prior art component combinations for making current mode amplifiers consists of single ended output operational amplifiers with feedback resistors operating in the shunt input mode to produce a low input impedance or the "current mirror" type of current mode amplifiers such as exemplified by the Motorola 3401P and the National Semi-conductor LM2900 single ended amplifiers. Such structure does not effectively produce ultra low input impedance amplifiers on the order of less than one ohm for efficient current mode applications requiring wide bandwidths and with response down to DC. Various other methods and structure for controlling input and output impedances and differential amplifier systems are disclosed in three of the inventor's previous U.S. Pat. Nos. 3,638,132, 3,845,404, and 3,955,149 which teach the use of active feedback in both the common and differential modes to improve differential amplifier performance involving high input impedances for voltage mode differential amplifiers, and current mode and differential mode amplifiers featuring both hardwire and optical coupling in the feedback systems.